Steel is an alloy of iron and carbon, and materials with a variety of characteristics can be produced as desired by controlling the carbon content and a working and heat treatment method. Accordingly, quantitative analysis of carbon in steel materials is an extremely important technique for controlling material characteristics.
From this point of view, techniques for analyzing the carbon content of, for instance, a billet or molten steel have been developed, and quantitative analysis of not more than 0.1% carbon in terms of mass % is now possible. For example, Patent Literature 1 describes a method enabling the carbon concentration in molten steel to be estimated accurately with an error of up to 10 ppm. Those techniques are used for accurately analyzing the carbon content of a material for the average composition.
Meanwhile, even with the same carbon content, mechanical characteristics of steel materials can be greatly changed depending on the condition of the working and heat treatment method. Such mechanical characteristics are determined by metallographic structure of iron and steel materials. The distribution and existence form of carbon in a micrometer or smaller scale play important roles in control of metallographic structure. Thus, the analysis of trace carbon in metallographic structure is extremely important.
Optical microscopes and electron microscopes are often used in analysis of metallographic structure. In particular, when an electron microscope is used, a metallographic structure with a size of less than 1 μm can be observed. Furthermore, a device for detecting characteristic X-rays that are generated from samples upon irradiation with incident electron beams can be installed on an electron microscope. Use of such a device makes it possible to perform qualitative and quantitative analyses of elements, such as carbon, composing metallographic structure and being distributed with a size of less than 1 μm.
In recent years, the structure control of steel materials has been highly advanced, and use of dual phase steel having a plurality of metallographic structures in a steel material is not unusual. Therefore, it becomes increasingly important to analyze, in addition to the average chemical composition of a steel material, information on a local chemical composition corresponding to metallographic structure, particularly the distribution of carbon.